Printing by thermal techniques of the kind here of interest is known in the prior art, as shown, for example, in U.S. Pat. Nos. 2,713,822 to Newman, 3,744,611 to Montanari et al, and 4,269,892 to Shattuck et al.
Various materials are employed as the major structural material of the resistive layer. Thus, for example, U.S. Pat. No. 4,103,066 to Brooks, et al is directed to polycarbonate resins and U.S. Pat. No. 4,269,892 to Shattuck, et al is directed to polyester resins and polyester and urethane resins. Both disclose conductive carbon black dispersed in the resin to provide a degree of electrical resistivity desired.
In certain thermal printing systems, resistance of the ribbon to permanent change from the heat of printing is sought. Particularly where the ribbon is to be reused, heat-resistance is critical since the ribbon must both retain its physical characteristics and not be significantly deformed as by stretching. Polyimide is known to be resistive to heat, and U.S. Pat. Nos. 4,236,834 to Hafer, et al and 4,253,775 to Crooks, et al teach reusable systems employing polyimide.
The teachings of both of these patents are prior art to the invention of this application. Both are directed to reusable elements for thermal printing for which this invention is also well suited. The Hafer, et al patent discloses a resistive layer of polyimide and carbon laminated to a thin aluminum layer. The Crooks, et al patent discloses a resistive layer "comprised of conductive particles, for example, of graphite, suspended in a high temperature polymer, for example, Kapton." Kapton is a brand name for a stable polyimide.
This invention also employs polyimide as the resin material of the conductive layer of a thermal transfer medium. In accordance with this invention, however, a blend of thermosetting and thermoplastic polyimides are employed to achieve, in addition to temperature stability, excellent electrical resistivity, as well as good strength and, where filled with graphite, excellent abrasion resistance.